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Influence of Blast Delay Time on Rock Fragmentation in a 22-ft Bench.

The U.S. Bureau of Mines is studying blast delay timing influences on rock fragmentaton in a series of tests that started in 3-ft concrete blocks, progressed to 45-in-bench reduced-scale tests in massive dolomite, and has now included 22-ft-bench full-scale tests in a thin-to-thick horizontally bedded dolomite. This paper discusses the full-scale test results and how they compared to the concrete block and reduced-scale studies. Six full-scale single-row blasts were shot at a 6-ft burden at delays of 2 to 48 ms, equivalent to 0.33 to 8.0 ms/ft of burden. All the blasted rock was screened from the first four shots. It was also established that sampling about 30 pct of the material adequatley predicted the total distribution. Optimum fragmentation occurred in shots delayed from 1 to 8 ms/ft of burden. The shot delayed at 0.33 ms/ft increased the 80-pct-passing size by 15 pct during the full-scale tests in the highly bedded rock, compared to a 40-pct increase for the same delay ratios in the reduced-scale tests in the massive dolomite. Each full-scale shot was filmed with high-speed cameras and instrumented with strain and pressure gauges. At optimum delays the arrival of gas-pressure-driven radial fractures interacted with stress waves emanating from the next hole, causing an increase in both strain level and duration.

Keywords

Mining-industry; Explosions; Explosives; Models; Simulation-methods

CODEN

PCETDN

Publication Date

19880101

Document Type

OP; Conference/Symposia Proceedings

Fiscal Year

1988

NTIS Accession No.

NTIS Price

Identifying No.

OP-70-89

ISSN

0732-619X

NIOSH Division

TCRC

Source Name

Proceedings of the 14th Conference on Explosives and Blasting Technique